The invention relates generally to ultrasonic sandwich transducers, and more particularly to such an ultrasonic sandwich transducer that is frequency selective.
Ultrasonic sandwich transducers known to date are wide-band short-distance transducers, as disclosed in U.S. Pat. No. 4,677,337 (EP O 154 706 A2) or EP 0 128 049, or as wide-angle proximity sensors. These transducers have high mechanical damping, i.e. low sensitivity, and are relatively expensive due to high material and production costs. Another disadvantage of previously known ultrasonic sandwich transducers is their narrow pattern directivity for certain frequency ranges. This is undesirable in applications for wide-angle detecting of glass breakage, such as that which occurs in motor vehicles during a break-in. Conventional transducers having a very large bandwidth can be used for this application. When a glass pane breaks, they pick up radiated ultrasonic vibrations and convert them into an electrical signal with frequencies corresponding to the frequencies of vibration. The corresponding frequencies of vibration that are typical when glass breaks make it possible for a glass pane break to be recognized by means of an evaluation circuit following the ultrasonic transducer. In this case, the evaluator must filter the frequencies corresponding to the glass break from the entire spectrum in which the ultrasonic transducer is sensitive. This may be accomplished by using filters since only the signals occurring at the concerned frequencies at break are relevant in determining a glass pane break. To simplify such an evaluation device of a glass break signalling configuration and at the same time to save costs, it would be advantageous to employ an ultrasonic sandwich transducer that is only sensitive to certain frequencies, such as the frequencies that occur when a glass pane breaks. One could then dispense with the filters, which would otherwise be needed in the evaluation circuit, or at least keep the number of filters to a minimum.
The invention is directed to the problem of developing an ultrasonic sandwich transducer that is highly sensitive to certain predetermined frequencies.